Method for bending tubular articles with a relative ratio of the bending radius and the outer diameter of the finished pipe which is less than 3.

Abstract

 Method for bending tubular articles (1a) with a relative ratio of the bending radius (R) and the outer diameter (D) of the finished pipe which is less than 3, comprising the following steps:
- forging an article (1a) made of material with a predefined composition depending on the final use and with a predefined length (L) in the longitudinal direction and an outer diameter (D);
- preparing a die (100) comprising:
· an asymmetrical bottom die half (110) with seat (111a) for the pipe (1a); and
· a top die half (120) with punch (121) asymmetrically arranged in the longitudinal direction (X-X) with respect to the vertical centre axis of the die;

- heating the pipe in an oven to the required temperature for forging depending on the specific composition of the material;
- positioning the article (1a) inside the bottom die half (110) in the longitudinal direction;
- relative closing of the two die halves (110,120) so as to start bending of the article (1a);
- opening the die;
- extracting the bent pipe.

Description

[0001] The present invention relates to a method and a die for bending tubular elements.

[0002] It is known in the technical sector relating to the manufacture of large-size tubular piping which is normally made by means of forging that there exists the need to obtain curved sections by means of bending of the pipe.

[0003] Although the bending methods performed for example using bending machines are per se conventional, these methods however are not suitable in the particular case where it is required to bend pipes which have relative ratios of the bending radius R and the outer diameter D of the pipe with values less than 3.

[0004] It is also known that this piping must have connecting branches extending in transverse - i.e. usually radial - directions and, particularly in the case of piping used for high-risk applications, such as nuclear power stations, these branches should not be welded to the respective pipe section after bending, but must be formed integrally therewith during forging of the pipe so as to ensure continuity of the piping and therefore avoid the risk of leakages of dangerous material.

[0005] The presence of these transverse branch extensions integral with the pipe means, however, that conventional bending machines cannot be used and in particular makes it difficult to perform bending of the pipe with the degree of precision required by the specific applications, said bending not being able to performed using normal known methods particularly in the case where the base of one or more of these branches is adjacent to the initial section of curvature of a curved section, said position making it particularly difficult to perform both forging of the article and subsequent bending.

[0006] The technical problem which is posed, therefore, is to provide a method and an associated die for bending pipes for piping having relative ratios R/D of the bending radius R and the outer diameter D of the pipe with values less than 3.

[0007] In connection with this problem it is also required that the die and method should be able to allow bending with the necessary tolerances also in the case where radial branches integral with the pipe are present during bending.

[0008] These results are achieved according to the present invention by a method according to the characteristic features of Claim 1 and a die according to the characteristic features of Claim 10.

[0009] Further details may be obtained from the following description of a non-limiting example of embodiment of the subject of the present invention provided with reference to the accompanying drawings in which:

• Figure 1 shows a side view of a pipe with radial branches, inserted inside a die according to the present invention;
• Figure 2 shows a front view of the assembly according to Fig. 1;
• Figure 3 shows a schematic cross-section along the plane indicated by III-III in Fig. 2;
• Figure 4 shows a side view of the assembly according to Fig. 1 during the first bending stage;
• Figure 5 shows a front view of the assembly according to Fig. 4;
• Figure 6 shows a schematic view along the plane indicated by VI-VI in Fig. 5;
• Figure 7 shows a perspective view of the assembly according to Fig. 1 during the final bending stage;
• Figure 8 shows a front view of the assembly according to Fig. 7;
• Figure 9 shows a cross-section along the plane indicated by IX-IX in Fig. 8;
• Figure 10 shows a side view of the assembly according to Fig. 7 in the direction of the arrow X shown in Fig. 8;
• Figure 11 shows a side view of the assembly according to Fig. 7 in the direction of the arrow XI shown in Fig. 8;
• Figs. 12a-12d shows schematic cross-sections along vertical planes of the pipe branch finishing stages; and
• Figs. 13-14 show side views of a practical example of application of the method according to the present invention.

[0010] As shown in Fig. 1 and assuming for the sole sake of convenience of description and without a limiting meaning a set of three reference axes in a longitudinal direction X-X, transverse direction Y-Y and vertical direction Z-Z, respectively, the pipe 1a to be bent extends in the longitudinal direction X-X and has an outer diameter D.

[0011] The straight article 1a used as the basic part for the method according to the present invention is obtained by forging a bloom of the material required and will have a form (produced at the steelworks), diameters, thicknesses and lengths which are suitable for obtaining the finished part with the required dimensions after forming/forging according to the present invention.

[0012] The forged article also has blocks 2a of material which project radially in specific zones and from which, using the methods described below, the integral branches 2 projecting radially from the surface of the finished pipe 1 will be obtained.

[0013] The rough forged pipe 1a is inserted inside an asymmetrical vertical die 100 formed by a bottom die half 110 and a top die half 120; the bottom die half (Fig. 3) has a seat 111 for the finished pipe 1, comprising a first straight section 111a, forming a first angle α with the horizontal base surface, and a second straight section 111b, forming an angle β with respect to the same base surface; the two straight sections are connected continuously by a suitable joining piece 111c.

[0014] The inner part of the curved top die half 120 has a form corresponding to the half curve which extends from the neutral axes N1 as far as the intrados I1 (Fig. 9) of the bent pipe 1.

[0015] If, as in the case shown, bosses 2a of material for forming the branches 2 are present, and said bosses 2a are arranged in the zones adjacent to the section to be bent, it is not possible to use a symmetrical bending process according to the conventional technique and maintain an active and reactive force along the same axis in the press, and it is instead necessary to perform asymmetrical bending.

[0016] For this purpose the top die half 120 has inside it a seat 122 suitably formed in the top die half 120 with dimensions corresponding to the dimensions of the boss 2a and a punch 121 asymmetrically arranged in the longitudinal direction X-X with respect to the vertical centre axis of the die so as to cause initial contact between the top die 120 and forged article 1a, along a section adjacent to the base 2b of the boss 2a and cause initial bending of the curve in the vicinity of the latter.

[0017] Using the die described above it is possible to implement a method for bending tubular elements, comprising the following steps:

• preparing a forged pipe 1a made of material with a predefined composition depending on the final use and with a predefined length L in the longitudinal direction and outer diameter D;
• said pipe may have at least one boss 2a with base 2b extending radially outwards;
• preparing a die (100) comprising:

  · an asymmetrical bottom die half 110 with seat 111a for the pipe 1a;

  · a top die half 120 with punch 121 asymmetrically arranged in the longitudinal direction X-X with respect to the vertical centre axis of the die;

  · and an optional seat 122 for receiving the boss 2a, if present;

• heating in an oven to the required temperature for forging depending on the specific composition of the material;
• ovalization of the longitudinal pipe section Lp corresponding to the section for bending by means of the press;
• positioning the straight article 1a inside the bottom die half 110 in the longitudinal direction X-X so that the base 2b of the boss 2a is substantially aligned in the vertical direction Z-Z with the head of the upper punch 121;
• starting bending of the article la by means of closing of the two die halves.

[0018] Although bending may be performed in a single step it is preferable to perform the final bending in several stages - preferably two or three stages - alternating with intermediate heating stages for restoring the initial forging temperature.

[0019] Should the die have a round cross-section it is possible to obtain a curved article already sized with a round cross-section; in practice it is preferred, however, to envisage slight ovalization of the bending die so as to allow a subsequent sizing step by means of which more precise tolerances may be determined; in this case the oval form of the die will be such that the larger axis is perpendicular to the plane which contains the axis of the article bent by means of the bending operation;

- machine-tool machining of the bent and cooled part so as to perform the finishing thereof to the final dimensions.

[0020] Should the bosses have a length sufficient for the purpose, it is possible to perform rough-machining, heat treatment and surface-finishing.

[0021] Where bosses 2a are present, as shown, should they have a length in the transverse direction Y-Y (= radial direction) less than the required length for the final branch 2, further steps are required in the method as described below with reference to Figs. 12a to 12d:

• boring the boss 2a in the radial direction;
• localized heating of the zone to be deformed, up to the temperature necessary for plastic deformation;
• multi-step extrusion by means of successive through-forcing of drift plugs 200 of increasing diameter until the external surface of the boss 2a makes contact with the internal surface of a suitably formed seat 315 in an external die 300;
• drawing the material by means of through-forcing of drift plugs with a diameter greater than the internal diameter of the branch, the thickness of which is reduced and the length of which increased to the design dimension;
• heat treatment required for use of the finished pipe;
• machine-finishing;
• surface finishing treatment (pickling, passivation and the like).

EXAMPLE

[0022] The accompanying drawings illustrate, purely by example, bending of a part performed during trial tests (Figs. 13 and 14) on a pipe sample with a diameter D = 515 mm and length L₁₁= 2,848 mm which had a first boss with a height H₁₁ asymmetrically arranged at distances L₂₁ = 1171.6 e L₃₁ = 1676.3 from the opposite ends of the pipe.

[0023] Said bosses had a height H₁₁ = H₂₁ = 200.8 mm, diameter of the base D_b = 377.8 and interaxial distance L₂₁ = 1014.5 mm (Fig. 13).

[0024] The pipe made of material 316L was treated with heating performed at T=950°C.

[0025] The finished and bent pipe had a length L₁ = 2640 mm, a first straight section with a length L₂₁ = 1014.5 mm, a second straight section L₃ = 460 mm, connected together by a joining piece with a radius of curvature R = 735.08 mm about 1.5 times the outer diameter D of the pipe and bending angle = 56°; the outer diameter D, after sizing, was between 497 and 502 mm, a first boss arranged in position L₂₁ = 1171.6 mm and a second boss arranged at a distance l₂₁ = 620 mm from the first boss; both bosses had a diameter of the base D_b = 377 mm and height h = 205 mm.

[0026] It is therefore clear how with the die and the method according to the invention it is possible to bend pipes using an industrial method which is repetitive and reliable also in the case of a relative ratio of the bending radius R and the outer diameter D of the pipe which is less than 3 and also form transverse branches integral with the pipe from radial bosses integral with the manufactured pipe at the time of bending.

[0027] The same method is also applicable in the particularly complex case of bending being started in a zone very close to the base of the branch, which case has not been solved by the prior art.

Claims

1. Method for bending tubular articles (1a) with a relative ratio of the bending radius (R) and the outer diameter (D) of the finished pipe which is less than 3, characterized in that it comprises the following steps:

- forging an article (1a) made of material with a predefined composition depending on the final use and with a predefined length (L) in the longitudinal direction and an outer diameter (D);

- preparing a die (100) comprising:

· an asymmetrical bottom die half (110) with seat (111a) for the pipe (1a); and

· a top die half (120) with punch (121) asymmetrically arranged in the longitudinal direction (X-X) with respect to the vertical centre axis of the die;

- heating the pipe in an oven to the required temperature for forging depending on the specific composition of the material;

- positioning the article (1a) inside the bottom die half (110) in the longitudinal direction;

- relative closing of the two die halves (110,120) so as to start bending of the article (1a);

- opening the die;

- extracting the bent pipe.

2. Method according to Claim 1, characterized in that it comprises the further step of machine-tool machining the bent and cooled part so as to obtain finishing to the final dimensions thereof.

3. Method according to Claim 1, characterized in that it comprises the further step of ovalization of the longitudinal pipe section (Lp) corresponding to the bending section before positioning of the article (1a) on the bottom die half (110).

4. Method according to Claim 1, characterized in that bending is performed by means of a multi-step sequence.

5. Method according to Claim 4, characterized in that before each bending step the pipe is heated to forging temperature.

6. Method according to Claim 1 for bending an article (1a) provided with bosses (2a,2b) projecting radially along the longitudinal pipe section to be bent, characterized in that it also comprises the steps of:

- preparing a seat (122) in the top die half (120), arranged adjacent to the punch (121) for receiving the said boss (2a,2b);

- positioning the straight article (1a) in the longitudinal direction X-X inside the bottom die half (110) in a position such that the base (2b) of the boss (2a) is substantially aligned vertically (Z-Z) with the head of the upper punch (121).

7. Method according to Claim 6, characterized in that it comprises the following further processing steps for the bosses:

- boring the boss (2a) in the radial direction;

- localized heating of the zone to be deformed, up to the temperature necessary for plastic deformation;

- introducing the boss (2a) inside a suitably formed seat (315) of an outer die (300);

- multi-step extrusion by means of successive through-forcing of drift plugs (200) of increasing diameter until the outer surface of the boss (2a) makes contact with the inner surface of a suitably formed seat (315) in an outer die (300);

- drawing the material by means of insertion of drift plugs with a diameter greater than the internal diameter of the branch until the design dimensions are achieved.

8. Method according to Claim 7, characterized in that it comprises the following further steps:

- heat treatment required for use of the finished pipe;

- machine-finishing;

- surface finishing treatment

9. Method according to Claim 1, characterized in that it comprises:

- a step for ovalization of the article (1a) during bending thereof; and

- a step for sizing of the pipe (1).

10. Die for bending tubular articles (1a) with a relative ratio of the bending radius (R) and the outer diameter (D) of the pipe which is less than 3, characterized in that it comprises:

- an asymmetrical bottom die half (110) with a seat (111) for the pipe (1a); and

- a top die half (120) with punch (121) asymmetrically arranged in the longitudinal direction (X-X) with respect to the vertical centre axis of the die.

11. Die according to Claim 10, characterized in that the top die half (120) has inside it a corresponding seat (122) with dimensions corresponding to the dimensions of a boss (2a) extending radially from the article (1a) and arranged adjacent to the said punch (121) in the longitudinal direction (X-X) so as to determine the initial contact between top die (120) and article (1a) along a section adjacent to the base (2b) of the boss (2a).

12. Die according to Claim 10, characterized in that the two die halves (110,120) are ovalized along the section of the article (1a) to be bent.

Drawing